1. Field of the Invention
The present invention relates to thermal-type infrared image sensing devices having their sensitivity in the far-infrared region.
2. Description of the Related Art
In a conventional infrared image sensing device, because, if the device temperature drifts, due to its output-signal level going outside the dynamic range, image sensing cannot be performed, the device needs to be kept at a constant temperature using a temperature-control mechanism. Therefore, in order to omit the temperature-control mechanism, reference-pixel elements each having no heat-insulated structure are formed other than pixel elements each having a heat-insulated structure; thus, by feeding back signals from the reference-pixel elements into a control circuit for driving the pixel elements, even if the device temperature is not controlled, the output level of the device has been made to be within a certain range, for example, as disclosed in page 12, and FIG. 1 of Japanese Patent Application Publication Laid-Open No. 2005-214639 (Patent Document 1).
A device, for example, has also been disclosed in page 3, and FIG. 6 of Japanese Patent Application Publication Laid-Open No. 2000-97767 (Patent Document 2), in which a light-shielding plate is provided on its package for shielding it from incident infrared light, and the light-shielded portion is used as reference-pixel elements.
In a general infrared image sensing device, in order to detect status change of the detector due to incident infrared light, voltage is applied to its pixel elements. For example, in the infrared image sensing device disclosed in Patent Document 1, voltage has been applied to each pixel element through its driving line. Therefore, caused by imaging-signal reading-out operations of the infrared image sensing device, self-heat generation has occurred in the pixel elements. The heat amount of the self-heat generation has been far much higher than a temperature rise due to infrared light incident from the exterior of the device.
In such an infrared image sensing device as disclosed in Patent Document 1, each reference-pixel element has been configured without a heat-insulated structure. Therefore, even when image sensing is performed without infrared light being incident from the exterior of the device, difference between output levels read out from the pixel element and the reference-pixel element occurs; accordingly, a signal outputted from the reference-pixel element has not been able to be used as a reference voltage for pixel signals.
The reason is as follows. The device has been configured in such a way that, for example, when the reference-pixel element has no heat-insulated structure, because the amount of heat transport (thermal conductance) from the detector of the pixel element to the image sensing device main body is significantly large, the reference-pixel element is easy to be cooled. Therefore, caused by the self-heat generation accompanying the operation of reading out the pixel signal, difference between the signal levels outputted from the reference-pixel element and the pixel element has occurred. Accordingly, a disadvantage has occurred in that a signal processing operation assuming this output difference must be considered.
The device can also be configured in such a way that, instead of the reference-pixel element having no heat-insulated structure as described above, the reference-pixel element does not absorb infrared light incident from the exterior, for example, that the infrared-light absorber is omitted. However, in such configuration, because infrared light is incident on the detector of the reference-pixel element, and an output signal is generated, a disadvantage has occurred in that the resultant voltage cannot be used as the reference voltage.
In the meantime, in the method disclosed in Patent Document 2 in which the reference-pixel-element portion is formed by providing the light-shielding plate on the package, the pixel element and the reference-pixel element can be formed to be exactly the same structure. However, because incident light is subject to diffraction due to the light-shielding plate, in order to prevent the effect, by isolating the pixel-element portion from the reference-pixel-element portion completely shielded from light, the portion on which light is incident and the portion completely shielded from the light need to be separated. Therefore, it has been disadvantageous to miniaturize the image sensing device.